On 26/06/2024 17:26, Lee Jones wrote:
> On Thu, 20 Jun 2024, Javier Carrasco wrote:
>
>> Use the struct_size macro to calculate the size of the tll, which
>> includes a trailing flexible array.
>>
>> Signed-off-by: Javier Carrasco <javier.carrasco.cruz@xxxxxxxxx>
>>
>> ---
>> The memory allocation used to be carried out in two steps:
>>
>> tll = devm_kzalloc(dev, sizeof(struct usbtll_omap), GFP_KERNEL);
>> tll->ch_clk = devm_kzalloc(dev, sizeof(struct clk *) * tll->nch,
>> GFP_KERNEL);
>>
>> Until commit 16c2004d9e4d ("mfd: omap-usb-tll: Allocate driver data at once")
>> turned that into the current allocation:
>>
>> tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
>> GFP_KERNEL);
>>
>> That has surprised me at first glance because I would have expected
>> sizeof(tll->ch_clk[nch]) to return the size of a single pointer, not
>> being equivalent to 'sizeof(struct clk *) * nch'.
>>
>> I might be missing/misunderstanding something here because the commit
>> is not new, and the error should be noticeable. Moreover, I don't have
>> real hardware to test it. Hence why I didn't mark this patch as a fix.
>>
>> I would be pleased to get feedback about this (why it is right as it is,
>> or if that is actually a bug).
>
> You don't need this H/W to test this our for yourself.
>
> Mock-up the structs in a user-space C-program and print out the sizes.
>
> Please report them all to justify the patch.
>
Values obviously depend on the architecture, but in general:
1.- Before commit 16c2004d9e4d:
1.1. tll = devm_kzalloc(dev, sizeof(struct usbtll_omap), GFP_KERNEL);
-> sizeof(struct usbtll_omap) = N
1.2 tll->ch_clk = devm_kzalloc(dev, sizeof(struct clk *) * tll->nch,
GFP_KERNEL);
-> sizeof(struct clk *) * tll->nch = M * nch
Total = N + M * nch,
where M is the size of a single pointer.
2.- After commit 16c2004d9e4d:
tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
GFP_KERNEL);
-> sizeof(*tll) = N
-> sizeof(tll->ch_clk[nch]) = sizeof(struct clk *) = M
Total = N + M
Therefore, it only allocates memory for a single pointer.
3.- struct_size (this patch):
sizeof(*tll) + nch * sizeof(struct clk *) = N + nch * M
What I meant with not having real hardware is that I could not replicate
the whole code with all their structures to get exact sizes, which don't
leave room for discussion or misunderstandings.
Best regards,
Javier Carrasco
>> ---
>> drivers/mfd/omap-usb-tll.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/mfd/omap-usb-tll.c b/drivers/mfd/omap-usb-tll.c
>> index a091e5b0f21d..5f25ac514ff2 100644
>> --- a/drivers/mfd/omap-usb-tll.c
>> +++ b/drivers/mfd/omap-usb-tll.c
>> @@ -230,8 +230,7 @@ static int usbtll_omap_probe(struct platform_device *pdev)
>> break;
>> }
>>
>> - tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
>> - GFP_KERNEL);
>> + tll = devm_kzalloc(dev, struct_size(tll, ch_clk, nch), GFP_KERNEL);
>> if (!tll) {
>> pm_runtime_put_sync(dev);
>> pm_runtime_disable(dev);
>>
>> --
>> 2.40.1
>>
>
